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ARMY RESEARCH LABORATORY
High Shear Strain-Rate Behavior of W-Ni-Fe Tungsten Heavy Alloy
Composites as a Function of Matrix Volume Fraction
ARL-TR-1694
by Tusit Weerasooriya andPaulMoy
Approved for public release; distribution is unlimited.
May 1998
The rmdings in this report are not to be construed as an official Department of the Army position unless so designated by other authorized documents. -
Citation of manufacturer's or trade names does not constitute an official endorsement or approval of the use thereof.
Destroy this report when it is no longer needed. Do not return it to the originator.
..
Army Research Laboratory Aberdeen Proving Ground, MD 21005-5066
ARL-TR-1694
High Shear Strain-Rate Behavior of W-Ni-Fe Tungsten Heavy Alloy Composites as a Function of Matrix Volume Fraction
Tusit Weerasooriya, Paul Moy Weapons and Materials Research Directorate, ARL
Approved for public release; distribution is unlimited.
May 1998
Abstract
The effect of matrix volume fraction on the high shear strain-rate deformation and failure behavior of W-Ni-Fe heavy alloys is studied using the torsional Hopkinson bar apparatus. High strain-rate tests (at 700/s) were conducted using torsion specimens made from W-Ni-Fe alloys with three different matrix volume fractions. Different matrix volume fractions were obtained by changing theW content in the W-Ni-Fe alloy while keeping W grain size approximately constant. Experimental observations indicate that as the matrix volume fraction is decreased, the strain to failure decreases, at high rates of loading. There were no significant changes seen in the deformation behavior of the three materials at high shear strain-rate loading used in this study.
ii
Table of Contents
Page
List of Figures........................................................................................................... v
List of Tables . .. ... .. .. ... .. ... ......... ... .. .... ... .... . ... . . . ... . . ... . .. ... . . .. ... . ......... .. . .. . .. ... .... ... .. . .... .. vu
1. Introduction....................................................... ... ....................... .......... ............... ... . 1
2. Experiments............................................................... .. ............................................. 3
2.1 Material................................................................................................................ 3 2.2 Specimen Geometry................................ .. .... ................................ ....................... 4 2.3 Quantitative Microstructural Analysis of the WHAs. .. .... .. . . ...... .. .... ... . .. . ... .. ........ 5 2.4 Test Apparatus . ..... ... .. ..... ...... ... .. ... ... .. . .. . ...... .. .. . . ..... .. .. .. ... .. ... ... . . .. . ... .. ........ .. ........ 6 2.5 Test Method ................................................................ :........................................ 7
3. Results and Discussion......................................................... ........................ ............ 7
4. Summary and Conclusions.................................... ............. ..................................... 12
5. References................................................................................. ................................ 15
Distribution List .............................................................. .... ..... ..... ... ............... ... ..... . 17
Report Documentation Page .................................................... ,.............................. 21
iii
INTENTIONALLY LEFT BLANK
lV
List of Figures
Figure
1. Microstructure of the WHAs Swaged to 17% Reduction in Area. (a) 90, (b) 93, and (c) 96% W, W-Ni-Fe Heavy Alloys .... ..... ................................ .......................... . 4
2. Dimensions and Tolerances of the Torsion Hopkinson Bar Specimen .................... . 5
3. Schematic of the Torsional Split-Hopkinson Bar ..................................................... . 7
4. Stress-Strain Curves up to Failure for 90% W WHA at 700/s .................................. . 8
5. Stress-Strain Curves up to Failure for 93% W WHA at 700/s .................................. . 8
6. Stress-Strain Curves up to Failure for 96% W WHA at 700/s .................................. . 9
7. Comparison of Shear Strains to Failure for WHAs With Three Different W Contents (Matrix Volume Fractions) . ....................... .. ............... ............ .................. . . 11
8. Shear Strains to Failure for WHAs With Three Different W Contents Plotted as a Function of Their Matrix Volume Fractions ............................... .............................. . 11
9. Typical Fracture Surface Morphologies From (a) High, (b) Intermediate, and (c) Low Matrix Volume Fraction WHAs ............... ..................... .............................. . 12
v
INTENTIONALLY LEFT BLANK
Vl
List of Tables
1. Chemical Composition and Mechanical Properties From Manufacturer ............... ... . 3
2. Quantitative Microstructural Data for the Three Alloys .......................................... .. 6
3. Yield Stress, Failure Strain, and Failure Stress Data From the High Strain-Rate Torsion Experiments .............................................................................. . 10
vii
INTENTIONALLY LEFT BLANK
Vlll
1. Introduction
In a composite such as the tungsten heavy alloy (WHA) examined here, the mechanical
properties are heavily dependent on the interfacial strength. WHAs are manufactured by the
sintering of W and matrix alloy powders (usually Ni and Fe). This produces a metal matrix
composite of pure W particles embedded in a W-Ni-Fe matrix. In a W-Ni-Fe composite, theW
W grain interfaces are far weaker than theW-matrix interfaces [1,2]. During shear (torsional)
loading, failure cracks are initiated at this interface by the maximum tensile component of the
applied shear load in the plane 45° to the shear loading [3]. Therefore, the mechanical properties
of WHAs are dependent on the W-W grain contiguity (defined as the fraction of the W-W
interfacial area to the total interfacial area). The initiated cracks at the W-W grain interfaces
typically propagate in the direction of the applied shear load, along the matrix, avoiding the hard
W grains. Therefore, at high rates of loading, the final failure is dependent not only on W-W
grain contiguity, but also on the matrix mean free path of the composite. To fully understand the
effect of microstructure on the failure behavior of WHA composites, it is necessary to study the
failure as a function of W-W grain contiguity as well as the matrix mean free path, which is a
measure of the width of the matrix material separating W-W grains.
Churn et al. [4] studied the effect of the microstructural parameter, the W-W grain
contiguity, on the quasi-static tensile and Charpy impact properties. They showed that the
ultimate tensile strength and elongation are not dependent on the W-W grain contiguity at the
lower rates of loading. However, the W-W grain contiguity parameter has been shown to
correlate the change in impact energy obtained from unnotched Charpy tests. These observations
were attributed to the lack of change in fracture morphology in quasi-static rate tensile tests and
an increase in ductile dimple-type fracture in the Charpy impact tests with decreasing contiguity.
Recently, under high shear strain-rate (torsion) loading conditions, failure behavior of a WHA
has been studied by Weerasooriya et al. [5] by only changing W-W grain contiguity while
keeping the matrix volume fraction (thus, the matrix mean free path width) constant. In this
controlled experimental study, it has been observed that the failure shear strain decreased with an
increase in W-W grain contiguity. This change in failure strain could be correlated to the change
observed in the fracture surface morphology: decreasing ductile dimples and increasing W-W
grain boundary facets as the W-W contiguity increased. However, as mentioned previously, the
Churn et al. study did not show any . change in quasi-static tensile elongation and fracture
morphology with change in contiguity.
1
German and coworkers studied the effect of W content on the tensile properties at quasi
static rates of loading [6,7]. In this case, tensile properties were influenced by both W-W grain
contiguity and matrix volume fraction, which influence the initiation and growth of the failure
crack, respectively. Since both these parameters vary with the amount of W in the alloy, the
reported influence of contiguity on the tensile properties reflects the influence of both the
contiguity and matrix volume. With the increase in W content, tensile elongation decreased.
The observed correlation was attributed to the formation of cracks in the W-W grain interfaces.
This result contradicts the observations by Chum et al., where only the contiguity was varied. At
quasi-static rates of loading, tensile failure strains remain unchanged as a function of contiguity
when the matrix volume fraction and W grain size are kept constant. However, failure strain
changes when both the matrix volume fractions are changed, resulting in variations in both the
contiguity and the matrix mean free path. At high rate shear loading and Charpy impact loading,
variations in contiguity alone change the strain to failure.
In the application of WHAs as kinetic energy (KE) penetrator materials, they are
subjected to high rates of loading. Previous work shows that the deformation and failure
characteristics of these alloys are dependent upon the rate of loading [3]. At high shear rates of
loading, WHAs can fail by shear localization. Therefore, to understand the effect of contiguity
and matrix volume fraction on the WHAs, which are to be used as KE materials, it is necessary
to determine their influence at high rates of loading.
The objective of this research is to investigate the effect of matrix volume fraction on the
deformation and failure behavior of W-Ni-Fe alloys at high shear rate loading. Matrix volume
fraction was varied by changing the W content in the alloy while keeping the W grain size
approximately constant and the Ni:Fe ratio at 7:3. WHAs of three different matrix volume
fractions with W weight contents of 90, 93, and 96% were used in this study. With a change in
W content, both the matrix volume fraction and the contiguity change. Specimens were tested to
failure at a high shear loading rate using a torsion Hopkinson bar apparatus. Adhering to careful
tolerances, the defect factor of the specimens was restricted to a small range. Defect factor of a
torsion specimen, defined by Andrews et al. [8], is a measure of geometric defect or variation in
the thickness along the gauge length. Restricting the defect factor of the specimens from the
three materials to a small range, the observed differences of the mean values of the deformation
and failure behavior of the materials will only depend on the microstructural differences in the
alloys. The observed differences in the deformation and failure properties in an alloy are due
mainly to the variations in the defect factor of the specimens of that material. Therefore, to
extract the effect of microstructure, the key is to machine the specimens with high tolerances so
that the variation of the failure properties within a material stays small. With the combination of
2
the experimental data from this program and the data from our previous work, where we varied
only the contiguity and kept the matrix volume fraction the same, one should be able to separate
the influence of contiguity (a measure of the initiation of failure) and matrix mean free path
width (a measure of the propagation of failure) on the high rate failure behavior. A future
publication will address this issue after the necessary additional experiments are conducted.
2. Experiments
2.1 Material.
The 17% swaged 90, 93, and 96% W alloys that were used for the experiments in this
report were obtained from Teledyne Advanced Materials. The nominal chemical compositions
and representative mechanical properties of these alloys are listed in Table 1.
Table 1. Chemical Composition and Mechanical Properties From Manufacturer
Chemical Composition Physical and Mechanical Properties
Alloy w Ni Fe Density Hardness UTS Elongatior (%) (%) (%) (vcm3
) (RJ (MPa) (%)
90W-7Ni-3Fe 90.06 6.93 3.02 17.12 37.10 1,091.0 17.9
93W -4.9Ni-2.1Fe 93.04 4.88 2.08 17.70 39.50 1,118.0 17.0
96W-2.8Ni-1.2Fe 96.10 2.79 1.11 18.40 39.30 1,151.5 10.0
Teledyne processed this alloy using the following procedure. An elemental mixture of
W, Ni, and Fe powders was cold isostatically pressed at 207 MPa (30,000 psi) in a drybag press.
The pressed material was then sintered in a flowing hydrogen atmosphere in a molybdenum
wound furnace at 1,520° C. The purpose of the hydrogen atmosphere was to reduce powder
surface oxides. The as-sintered material was vacuum-annealed at 1,000° C for 10 hr to remove
the absorbed hydrogen. The annealed material was then heated in an inert gas atmosphere to
1,100° C, soaked for 1 hr, then water-quenched to give better dynamic impact properties. The
bars were then machined and swaged to a 17% reduction in area.
3
Figure 1 shows the microstructure of these alloys taken in the longitudinal direction. The
microstructure in the transverse direction is similar to that in the longitudinal direction. Swaging
up to 17% does not seem to affect the morphology of the microstructure. The microstructure
consists of two phases: the spherical grains of nearly pure W with a bee crystal structure, and a
W-Ni-Fe matrix of fcc crystal structure with an approximate composition of 50Ni-25Fe-25W.
The relatively brittle W grains are approximately 25-28 mm in diameter. Most of the W grains
are surrounded by a thin layer of matrix material, which gives the ductility to the composite.
However, some W grains are in contact with adjacent W grains; these areas of contact increase
with theW content of the alloy. More detailed quantitative analysis of the microstructures of
these three alloys is given in a later section in this report.
Figure 1. Microstructure of the WHAs Swaged to 17% Reduction in Area. (a) 90, (b) 93, and (c) 96% W, W-Ni-Fe Heavy Alloys.
2.2 Specimen Geometry.
The geometry of the torsional test specimen is shown in Figure 2. The gauge section of
the test specimen is a thin wall tube (0.38-mm wall thickness) of 2.540-mm gauge length and
outside and inside diameters of 10.16 and 9.40 mm, respectively. The wall thickness
corresponds to an average of 14 W grains. Hexagonal flanges with 60° shoulders are machined
at both ends of the thin tubular gauge section and are used to attach the specimen to the elastic
input and output bars of the test system. After initially machining the inside and outside
diameters of the gauge section to 0.25 mm undersize and oversize, respectively, the specimens
were machined to the final dimensions with the specified tolerances by honing the inside and
grinding and polishing the outside. Grinding is also employed to obtain a radius of
approximately 0.38 mm between the shoulder and the gauge area. With the careful machining of
4
the specimens to the specified tolerances, the defect factor of the gauge area of all the specimens
is forced to lie within a small range.
I X
I I I L "~.9mm ~
-·-·Ee~- ~ 14.2875mm I 10.67mm
~I 1
14.2621mm HEX
I SECTION X-X X
DIM mm
c 25400 D 10.1600 ± 0.0127 DIA E 9.3980 ± 0.0127 DIA
Figure 2. Dimensions and Tolerances of the Torsion Hopkinson Bar Specimen.
Using this short gauge length specimen with 60° shoulders, an almost homogeneous state
of strain is achieved after a few reflections of the applied shear stress pulse. In a specimen with
the same gauge area dimensions, but with end flanges with sharper (90°) shoulders, the plastic
zone starts at the flange-gauge section interface. Although the plastic zone starts at this
interface, it spreads gradually through the specimen and engulfs the whole gauge section [9].
2.3 Quantitative Microstructural Analysis of the WHAs.
Quantitative microstructural values, matrix volume fraction, W-W grain contiguity,
matrix mean free path width, and W grain size for the three alloys are obtained by image analysis
of the optical micrographs. Digitally scanned images of the micrographs were analyzed using
Image 1.47 software from the National Institutes of Health on a Macintosh computer. For each
WHA, micrographs from five different locations are used for the image analysis. Table 2 gives
the results of the quantitative microstructural analysis with standard deviations. The matrix
volume fractions, defmed as the ratio of the total area of the matrix and the total area of the
micrograph, of the 90, 93, and 96% W content WHAs are 0.23, 0.14, and 0.09, respectively.
One would expect, as the W content increases, matrix volume fraction decreases. Matrix mean
free path width is defined as the total matrix area divided by two times the difference between
the total length of W grain perimeters and the total length of W-W grain contacts. This width
defines the average thickness of the matrix material in a WHA micrograph; the failure crack
5
would propagate through this matrix material. For the three alloys of 90, 93, and 96% W, the
average matrix mean free path widths are 0.81 , 0.57, and 0.45 ~m, respectively, decreasing with
increasing W content. W-W grain contiguity in Table 2 is defined as the ratio of the total length
of W-W grains in contact with each other to the total circumferential lengths of the W grains in
the micrographs. The W-W grain contact is the weakest interface and therefore is the most
probable failure initiation location. Using this definition of contiguity, 90, 93, and 96% W alloys
have mean contiguities of 0.15, 0.29, and 0.41 respectively. W grain sizes are calculated as the
square root of four times the total W grain area divided by the value of 1t times the total number
of W grains. The grain sizes for the three materials are very similar, with the average grain sizes
ranging from 24.48 to 28.14 mm.
Table 2. Quantitative Microstructural Data for the Three Alloys
90W-7Ni-3Fe 93W-4.9Ni-2.1Fe 96W-2.8Ni-1.2Fe
Microstructural Parameters Mean Std. Mean Std. Mean Std. Deviation Deviation Deviation
Matrix Volume Fraction 0.23 0.03 0.15 0.02 0.09 0.03
W-W Grain Contiguity 0.15 0.05 0.29 0.04 0.41 0.07
Matrix Mean Free Path Width (mm) 0.81 0.11 0.57 0.04 0.45 0.12
W Grain Size (mm) 25.23 4.16 28.14 2.07 24.48 2.43
2.4 Test Apparatus.
The high rate tests were conducted using a torsional split-Hopkinson bar. The original
apparatus was developed by Kolsky to conduct compression tests at high loading rates. This
apparatus was later modified to conduct torsion tests by Baker and Yew [1 0] and Duffy et al.
[11]. The torsional Hopkinson bar used to conduct these experiments was based on the apparatus
developed by Duffy et al. [12]. A schematic of the torsional Hopkinson bar is given in Figure 3.
It consists of two 7075-T6 aluminum bars of diameter 25.4 mm and length 2,438 mm. The
hexagonal flanged thin wall specimen is attached between the two bars. A torque is stored
between the non-specimen end of the input bar and the clamp. The high strain-rate of loading is
applied to the specimen by the sudden release of the stored torque by breaking the clamp. The
position of the clamp on the input bar relative to the loading pulley determines the duration of
the stress pulse. This incident torsional stress pulse travels toward the specimen after its release;
at the specimen, part of the pulse transmits through the specimen to the output bar, and the
remainder reflects back to the input bar. From the incident, reflected, and transmitted pulses
6
measured using strain gauges mounted on the input and output bars, the stress, strain, and strain
rate can be inferred as a function of time. The details of the data acquisition and reduction
procedure are similar to those given by Weerasooriya [13]. When a shear band initiates, the
strain and strain rate that are determined by this method represent average values in the gauge
section of the specimen.
OUTPUT BAR
BEARINGS REDUCER Bl-B8
& MOTOR
I
Figure 3. Schematic of the Torsional Split-Hopkinson Bar.
2.5 Test Method.
LOADING PULLEY
I
High rate torsion tests were conducted on specimens from the 90, 93, and 96% W heavy
alloys using the torsional Hopkinson bar apparatus. All the tests were conducted to failure at an
average strain-rate of approximately 700/s. Data from the tests were reduced to obtain the stress
strain behavior of the material and the failure stress and strain.
3. Results and Discussion
The reflected and transmitted stress pulses measured in each of the tests are reduced to
obtain the stress-strain plots shown in Figures 4, 5, and 6. The method used to reduce data is
described elsewhere [ 13]. The initial transients in the stress-strain plots are artifacts of the high
rate testing method.
Reduced stress-strain data from the specimens from 90% W WHA are shown in the
Figure 4. The stress-strain data show that prior to an average total shear strain of 0.144, the
material work-hardened. After that point (instability strain), material seems to soften with
increasing strain.
7
1500~--~----~--~----~--~----~
,_ co Q) ..c (f)
0
··-------------~-- - - - - -------- -- --~-...... ---..... ---~ -------.--------.. ; .... --------------
- - -~----- · ---·-··· · · - - ; ...... ------------I-----------··· ----1---------------- - -~------ - - ----------
___ -----··>···--·--·----··-··l·-··--·--··-···-··L·---·----------·r-····-··--··---·-l·-····-·········-·
······---·[···········-·····T················i·············-···-i··················'··················
0.00 0.05 0.10 0.15 0.20 0.25 0.30 Shear Strain
Figure 4. Stress-Strain Curves up to Failure for 90% W WHA at 700/s.
Figure 5 shows the reduced stress-strain data at 700/s strain-rate from 93% W WHA
specimens. As in the 90% W material, the material began work softening with increasing strain,
beyond an average strain of 0.11.
Figure 6 shows the reduced stress-strain data at 700/s strain-rate from 96% W WHA
specimens. This material starts to work-soften at shear strains above a mean strain of 0.105.
;f1200 ~
(f) (f) Q) ,_
Ci5 ,_ co Q) ..c (f)
Shear Strain
Figure 5. Stress-Strain Curves up to Failure for 93% W WHA at 700/s.
8
1500~----------------------------~
'i 1200 ~ -~ 900 ~
U5 Gl 600 Q) .c CJ)
300
- -·t··············-·r··············- ·····------------- ···········-- ----·--······----------
··---··: ·······--··---·--···--·-·········----; .................. : .................. ; ................. .
0.15 0.20 0.25 0.30 Shear Strain
Figure 6. Stress-Strain Curves up to Failure for 96% W WHA at 700/s.
Table 3 shows a comparison of failure strains, failure stresses, yield stresses, and strains
at the maximum stress (instability strain) for the three alloys. Because of the initial ringing in the
stress-strain data, the reported yield stresses are determined from extrapolating the stress-strain
curves through the initial overshoot back to the linear portion and then taking a 0.2% offset from
the linear portion. The material with highest W content, the 96% W alloy, has a mean
extrapolated yield strength of 1128.7 MPa; and the yield strengths of 93 and 90% W alloys are
1023.8 and 1056.5 MPa, respectively. The dynamic yield stress does not appear to be dependent
on the W content, and hence on the matrix volume, with any statistical significance. Mean
failure strengths of the material with 96, 93 and 90% W contents are 1069.0, 983.9, and
1003.9 MPa, respectively. There is no statistically significant trend in failure strength as a
function of matrix volume as well. Mean failure strains for the 96, 93, and 90% alloys are 0.16,
0.21, and 0.26, respectively; their respective standard deviations are 0.02, 0.03, and 0.02.
Variations in failure strain within one material is attributed to the variation in the defect factor of
the specimen gauge section (function of the tolerances used in machining of the specimen). But
the variation in the mean values of the failure strain between different materials is due to the
differences in the material microstructure. Failure strains of all the specimens from the three
alloys are shown in Figure 7. Lower matrix volume fraction materials failed earlier than the
higher matrix volume fraction materials with a very high probability (a statistically insignificant
probability of 0.0003 for 90 and 93% W material to fail at the same level of strain, and a
statistically insignificant probability of 0.0005 for 93 and 96% W materials to fail at the same
level of strain). Therefore, it can be concluded with certainty that as the matrix volume fraction
9
of W-Ni-Fe heavy alloy increases, the failure strain under high shear strain-rate loading also
increases. As seen in Figure 8, the relationship between mean failure strain and the matrix
volume fraction is linear (correlation coefficient, R = 0.99853). Instability strains for the three
materials are approximately the same and therefore are not dependent on matrix volume fraction.
Table 3. Yield Stress, Failure Strain, and Failure Stress Data From the High Strain-Rate Torsion Experiments
Yield Stress Failure Strain Failure Stress Strain@ Maximum Alloy (MPa) (MPa) Stress
(matrix volume fraction) Mean
Std. Mean
Std. Mean
Std. Mean
Std. Deviation Deviation Deviation Deviation
90W-7Ni-3Fe (0.23) 1056.5 29.5 0.26 0.02 1003.9 36.9 0.14 0.02
93W-4.9Ni-2.1Fe (0.15) 1023.8 46.5 0.21 0.03 983.9 37.3 0.11 0.01
96W-2.8Ni-1.2Fe (0.09) 1128.7 44.8 0.16 0.02 1069.0 43.6 0.11 0.01
From our previous work, it has been found that the W-Ni-Fe heavy alloy torsion
specimens fail by the initiation and growth of a shear localization [3]; the localized instability
initiated by the opening of the cavities at W-W grain interfaces and then grew along the matrix.
The reason for the early shear failure of the lower matrix volume fraction material is the presence
of larger W-W grain interfaces and earlier failures at these interfaces. Cracks form easily at
these interfaces. Separated W-W grain interfacial cracks act as the stress concentrators for an
early localized shear failure. In our earlier high rate torsion work on the unheat-treated material,
the fracture surfaces of the failed specimens reveal W-W grain interface failure. When theW
grains are separated with a thin layer of matrix material, as in the case of a low contiguity
material, the initial cracks have to start at the matrix-W grain interface or at the remaining small
W-W grain interface. It takes a larger strain to initiate cracks at this W -matrix interface; the
cracks that initiate at the remaining smaller W-W grain contacts will naturally be much smaller
in size and therefore have a smaller stress intensity factor than the cracks in a higher contiguity
material where the crack lengths formed at W-W grain separation are larger. Thus, the cracks
started in a lower contiguity material require a larger strain to propagate.
10
0.30----~---~---~----
~ ~ + 0.25 ------ ------····----- ------:X----- --------- ---- ----- --------~- ----- ----------- ---·-------------
-~ ~ + ] 0.20 --············ ·············• ·· ················ ··········!································•···································
·- + ~
~ . . : ;
0.15 - -------------------- ................. ......................... .. + .................................. ; ................................. ..
0.10~-------·--------~·---------i' ------~ 90 (0.23) 93 (0.15) 96 (0.09)
% of Tungsten (Matrix Volume Fraction)
Figure 7. Comparison of Shear Strains to Failure for WHAs With Three Different W Contents (Matrix Volume Fractions).
0.26 r-------------~------~.....,
0.24
!:: 0.22 -~ r/1 ~ 0.20 ~ -·-~ ~ 0.18
0.16
. ' I - . - I : : : . :
---- - --- ----- -'--- - -- - -----------~-----------------'-----------------L------- -- --·j·-----------------r-----------------i-----------------
: : :_ . :_
........... ---~-···········-··· .. f ................. :---·.. . -----····: ----- ..... --- --·-· ..... ...... -· t··- .......... ----~-- - - · ........... .
--------------j------------------i ................................................................ _______ , _______ __________ , __ ______________ _
· : . -. -
....... -----i------ -- - -------- -:-----------------· - --··---- ---------~----------------- · ------- ----------t --- ----- ---------:------ --- - --- -- --
: : : . . . ~ ~ ~ 0.14 ..__...__...__...__..___..___..___..______.
0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22 0.24
Matrix Volume Fraction
Figure 8. Shear Strains to Failure for WHAs With Three Different W Contents Plotted as a Function of Their Matrix Volume Fractions.
11
Figure 9 shows typical fracture surface morphologies from specimens from the three
materials with differing matrix volume fractions. The fracture surface of the specimens from
low matrix volume fraction material consists of brittle cleavage fracture surfaces of W grains,
brittle W-W interfacial separation zones, and typical smooth shear localization zones as shown in
Figure 9(c). In addition to these types of fracture zones, ductile dimple-type separation zones of
matrix material are also present in the specimens from higher matrix volume (Figure 9 [a and b]).
This is expected due to the presence of lesser amount of W-W grain interfacial zones in the
material with higher matrix volume fractions.
Figure 9. Typical Fracture Surface Morphologies From (a) High, (b) Intermediate, and (c) Low Matrix Volume Fraction WHAs.
4. Summary and Conclusions
The effect of matrix volume fraction on high shear strain-rate deformation and failure
behavior of three W -Ni-Fe heavy alloys were studied using a torsional Hopkinson bar apparatus.
High strain-rate tests (at 700/s) were conducted to failure on torsion specimens made from
materials with three different W contents: 90, 93, and 96%. After the tests, all the fracture
surfaces of the specimens were analyzed using a scanning electron microscope.
At the loading rate used in the tests, increasing the matrix volume fraction increases
strain to failure linearly. There are no significant differences in the deformation behavior (flow
stress) of the three materials. The observed increase in failure strain with increasing matrix
volume fraction is attributed to the decrease in zones of brittle failure mode. In the specimens
from the material with lowest matrix volume fraction, the fracture surface of the specimens
shows cleavage of W grains, brittle failure at the W-W grain interfaces, and smooth zones,
12
indicating shear localization. There were dimple-type ductile matrix fracture zones in the
material from higher matrix volume fractions in addition to the cleaved W grains, brittle W-W
interface separation zones, and smooth shear localized zones.
13
lNTENTIONALL Y LEFT BLANK
14
5. References
1. Chum, K. S., and D. N. Yoon. "Pore Formation and its Effect on Mechanical Properties in W -Ni-Fe Heavy Alloy." Powder Metallurgy, vol. 22, pp. 175-178, 1979.
2. Ekbom, L. "Influence of Microstructure of Liquid-Sintered Tungsten-Base Composites on the Mechanical Properties." Scandinavian Journal of Metallurgy, vol. 5, pp. 179- 184, 1976.
3. Weerasooriya, T., and P. A. Beaulieu. "Effects of Strain-Rate on the Deformation and Failure Behavior of 93W-5Ni-2Fe Under Shear Loading." Material Science and Engineering, vol. A172, pp. 71-78, 1993.
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13. Weerasooriya, T. "MTL Torsional Hopkinson Bar." MTL TR 90-27, U.S. Army Materials Technology Laboratory, Watertown, MA, 1990.
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High Shear Strain-Rate Behavior of W -Ni-Fe Tungsten Heavy Alloy Composites as a Function of Matrix Volume Fraction PR: lL162618AJf80
6. AUTHOR(S)
Tusit Weerasooriya and Paul Moy
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION REPORT NUMBER
U.S. Army Research Laboratory ATTN: AMSRL-WM-TD ARL-TR -1694
Aberdeen Proving Ground, MD 21005-5066
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13. ABSTRACT (Maximum 200 words)
The effect of matrix volume fraction on the high shear strain-rate deformation and failure behavior of W -Ni-Fe heavy alloys is studied using the torsional Hopkinson bar apparatus. High strain-rate tests (at 700/s) were conducted using torsion specimens made from W-Ni-Fe alloys with three different matrix volume fractions. Different matrix volume fractions were obtained by changing theW content in the W-Ni-Fe alloy while keeping W grain size approximately constant. Experimental observations indicate that as the matrix volume fraction is decreased, the strain to failure decreases, at high rates of loading. There were no significant changes seen in the deformation behavior of the three materials at high shear strain-rate loading used in this study.
14. SUBJECT TERMS 15. NUMBER OF PAGES
tungsten heavy alloy; matrix volume fraction; high strain rate; torsion; failure 23 16. PRICE CODE
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